I totally missed what this was at first. He's some detail if you haven't watched the teaser yet:
"During the WGA strike Joss Whedon started writing a three part musical series for the internet. Each of the three episodes will be approximately ten minutes each. Co-writers for the internet feature are Joss’ brothers Zack and Jed and Jed’s Fiancé Maurissa Tancharoen .
“It’s the story of a low-rent super-villain, the hero who keeps beating him up, and the cute girl from the laundromat he’s too shy to talk to.” says Whedon.
“Doctor Horrible’s Sing-Along Blog” will star Neil Patrick Harris as Dr. Horrible, Nathan Fillion as Captain Hammer, Felicia Day as Penny and a cast of dozens.”